Unit tests are very valuable in helping developers find problems early, facilitate change, simplifies integration and even act as standalone documentation. The most widely used frameworks on the front-end is QUnit.

JavaScript Code Quality Tools

Any code base eventually becomes huge at some point, and simple mistakes can become show stoppers and waste hours of debugging. This is when static code analysis tools come into play and help developers to spot such problems. Most popular choices are JSHint, JSLint, etc.

CSS Frameworks

CSS framework can speed up development, make your site more aesthetically pleasing, and help eliminate some of the cross-browser compatibility issues. Popular framework in this category include 960 Grid System, Blueprint, etc.

Build tools

Using one or more build tools has many advantages like accelerating development, elimination of redundant tasks, having a history of builds and releases in order to investigate issues and more. Some of the most important tools in this category are Make, Apache Ant, Grunt, or even custom shell scripts.

Head-less Browser Testing

A headless browser is a web browser without a graphical user interface and it can be very useful for automating webpage testing. A popular example is PhantomJS, etc.

These languages usually extend CSS with dynamic behavior such as variables, mixins, operations and functions. Some examples are LESS, Sass, etc.

JavaScript shims, fallbacks, and polyfills

These are JavaScript libraries that replicate the standard API found as natively in the new browsers, for the older browsers. A comprehensive list of shims, fallbacks, and polyfills can be found at the Modernizr site.

Webapps for testing JavaScript / CSS snippets

These webapps are designed to help JavaScript and CSS developers test snippets of code, within some context, and debug the code collaboratively. Examples are JS Bin, JsFiddle, Dabblet, etc.

Behavior-driven development (or BDD) is an agile software development technique devised by Dan North, and is based in the convention that unit test names are whole sentences starting with the word should and they are written in order of business value. A widely used framework that supports it is Jasmin.

Cloud debugging/testing services

Setting up your own testing infrastructure can be time consuming, so companies like Saucelabs offer their infrastructure as a service.

Automated screenshot services

These services makes screenshots of your web design in different operating systems and browsers, for testing purposes. A popular example is Browsershots.